Estimate of the background for the x-ray microcalorimeter Spectrometer onboard of IXO

We present a study of the background for the array of microcalorimeters onboard of the International X-ray Observatory space mission. We investigated through simulations the rates at the focal plane of soft and hard particles in L2 orbit. Assuming the presence of an anticoincidence instrument, we derived an estimate of the residual background. The preliminary results reported in this paper are based on a number of simplifications of the actual picture. Efforts to improve the model are on-going.